This invention concerns novel solvents for dissolving polymers containing tetrafluoroethylene (particularly the homopolymer, PTFE) which have a relatively high melting point. The solvents dissolve such polymers more rapidly, and/or the solutions are more stable, than previously disclosed solvents, which are advantages in equipment utilization and other manufacturing costs, and/or the purity of the resulting solution and any product made from it. For example, the instant solvents dissolve the TFE polymers more rapidly than perfluorinated aliphatic compounds, and do not discolor (at the required elevated temperatures) as perfluorinated aromatic solvent solutions may do.
U.S. Pat. No. 4,360,388 discloses certains solvents for TFE polymers, including perfluorodecalin, perfluoromethyldecalin, perfluorodimethyldecalin, perfluoromethylcyclohexane and perfluoro(1,3-dimethylcyclohexane). All of these solvents are believed to have critical temperatures below 340.degree. C., and hence are not solvents for PTFE.
B. Chu, et al., in a series of papers [Macromol., vol. 20, p. 702-703 (1987); Macromol., vol. 21, p. 397-402 (1988); Macromol., vol. 22, p. 831-837 (1989); J. Appl. Polym. Sci., Appl. Polym. Sym., vol. 45, p. 243-260 (1990)] describe the measurement of the molecular weight of poly(tetrafluoroethylene) (hereinafter sometimes PTFE) in solution. The solvents used in these studies were perfluorotetracosane and oligomers of poly(chlorotrifluoroethylene).
P. Smith and K. Gardner, Macromol., vol. 18, p. 1222-1228 (1985) review and discuss both the practical and theoretical aspects of dissolving PTFE. As reported by them, PTFE has been dissolved only in perfluorokerosenes and perfluorinated oils, in other words, perfluorinated higher molecular weight alkanes. They report that PTFE will not dissolve in perfluorodecalin, octafluoronaphthalene or decafluorobenzophenone. There is no discussion in this paper on the rates of dissolution of PTFE in solvent.
U.S. Pat. No. 3,461,129 reports in Example A that 4-ethoxy-2,2,5,5-tetrakis(trifluormethyl)-3-oxazoline dissolves low-melting (83.degree.-145.degree. C. melting point) PTFE. There is no mention of dissolution of higher melting PTFE.